(1) Field of the Invention
This invention relates to a radiation detector having scintillators, a light guide and photomultiplier tubes arranged in the stated order and optically combined to one another, and to a method of manufacturing the radiation detector.
(2) Description of the Related Art
This type of radiation detector is used in a medical diagnostic apparatus such a PET (Positron Emission Tomography) apparatus or a SPECT (Single Photon Emission Computed Tomography) apparatus for detecting radiation (e.g. gamma rays) released from radioisotopes (RI) introduced into a patient and accumulated in a region of interest, and obtaining sectional images of RI distribution in the region of interest. The radiation detector includes scintillators that emit light in response to incident gamma rays released from the patient, and photomultiplier tubes for converting the light emitted from the scintillators to pulsed electric signals. An earlier radiation detector had the scintillators and photomultiplier tubes arranged in a one-to-one relationship. In recent years, a technique has been employed to combine a plurality of scintillators to photomultiplier tubes smaller in number than the scintillators. With this technique, positions of incidence of gamma rays are determined from power ratios of the photomultiplier tubes to enhance resolution. A construction of a conventional radiation detector will be described hereinafter with reference to the drawings.
FIG. 1 is a schematic view showing an outward appearance of a conventional radiation detector. FIG. 2 is a section taken on line 100-100 of FIG. 1. FIGS. 1 and 2 show an example disclosed in Japanese Patent Publication No. 06-95146 (1994). This radiation detector RDA includes a scintillator array SA, a light guide LA optically combined to the scintillator array SA, a plurality of (four in FIGS. 1 and 2) photomultiplier tubes K1, K2, K3 (not seen in the figures) and K4 optically combined to the light guide LA. The scintillator array SA is an aggregate of scintillators S divided by numerous light reflecting elements DA inserted peripherally thereof. The scintillator array SA may be surrounded by light reflectors (not shown).
With this radiation detector RDA, the light guide LA is formed of an optically transparent material defining numerous slits MA of predetermined depths cut by a dicing saw or wire saw. The slits MA have optical elements (e.g. light reflecting elements or light transmitting elements) inserted therein. The slits MA have larger lengths from inner to outer positions of the light guide LA. This construction adjusts quantities of light from the scintillators S distributed to the four photomultiplier tubes K1-K4 to discriminate positions of incidence of gamma rays.
The conventional radiation detector RDA noted above has the following drawbacks.
The radiation detector RDA is a high-resolution detector using the scintillators S of high sensitivity as proposed in recent years, and the scintillator array SA has far more scintillators than the scintillator array of an earlier detector. Consequently, each scintillator S has a smaller section than a scintillator in the earlier detector. Generally, the smaller scintillators S provide, by absorption or diffusion, the lower probability of photons produced inside moving into the light guide LA. This reduces the capability of discriminating, and thus detecting, positions of incidence of gamma rays.
Where the scintillator array SA is designed such that individual scintillators S surrounded by numerous light reflecting elements DA, great numbers of scintillators S and light reflecting elements DA are required. This will results in a complicated manufacturing process, and thus high cost.
When suitable light reflecting elements DA are inserted in the slits MA after a shaping process, gaps are formed between the reflecting elements DA and slits MA, thereby lowering reflection efficiency also. As these factors reduce output by incident gamma rays to make an accurate discrimination of positions impossible, an overall image quality will also deteriorate.
More particularly, a reduced discriminating ability results in a reduction in resolution. Where such radiation detector RDA is used in a medical diagnostic apparatus such as a PET apparatus or SPECT apparatus, images obtained by the apparatus will have poor quality. When a region of interest is a tumor, for example, the tumor may not be accurately outputted on the image.
Further, the light reflecting elements DA or light transmitting elements inserted or filled as optical elements between the scintillator S make an accurate discrimination of positions even more difficult, particularly where light transmitting elements are used. That is, it is common practice to use a highly transmissive optical adhesive to form the light transmitting elements. However, since the optical adhesive forms adhesive layers, it is difficult to control the thickness of the light transmitting elements. As a result, variations will occur in the thickness of the light transmitting elements, so that the scintillators are not arranged at equal intervals. This further encumbers an accurate discrimination of positions of gamma ray incidence.